Friction stir welding joints of 2195-T8 Al–Li alloys: Correlation of temperature evolution, microstructure and mechanical properties

Abstract

7.5 mm thick 2195-T8 Al–Li alloy plates were manufactured by friction stir welding (FSW). Thermal history of different regions in welding joints was recorded during welding. Microstructure characterization was conducted out by electron back scattered diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Mechanical properties were measured using micro-hardness and tensile test. The correlation of temperature evolution, microstructure and mechanical properties of individual regions in welding joints was discussed. It was shown that peak temperature was 358 °C–376 °C in the heat affected zone (HAZ) where joint failure occurred due to the dissolution and coarsening of T1 and development of coarse grain boundary phase and precipitate free zones (PFZs). Peak temperature of thermo-mechanically affected zone (TMAZ) ranged from 376 °C to 401 °C, leading to complicated precipitation behavior. The dissolution and coarsening of precipitates resulted in joint softening, while a layer of fine equiaxed grains at the interface of the AS-TMAZ/NZ increased the hardness of this region. The base material (BM) mainly consisted of rolling textures, while the NZ and TMAZ had strong shear textures. Moreover, the intensity of shear textures was evidently higher in the NZ, but relatively lower in the RS-TMAZ. The hardness and tensile strength profiles of welding joints appeared in “W” shape. From the base material zone to the nugget zone within a welding joint, fracture mechanism changed from brittleness fracture to toughness fracture.